The number of people who are deaf and who receive bilateral cochlear implants (CIs) has grown substantially in recent years. Many bilateral CI users demonstrate significant benefits from bilateral implantation, such as better speech-understanding in noise and improved sound localization. However, the performance of many patients is still notably worse than that of normal hearing (NH) listeners. Proposed experiments will systematically investigate a number of factors that are likely to contribute to the gap in performance. We will use a multi-pronged approach to integrate a number of research methodologies that are concerned with spatial and binaural hearing. Methods will vary from having tight control over the signals presented to the auditory nerve to difficult-to-control stimuli presented in free field. Within this range, using novel binaural research processing strategies, we will manipulate the stimulus naturalness, from unrealistic non-speech stimuli, such as electrically pulsed signals presented to single pairs of electrodes, to the most realistic of stimuli, speech presented under complex multi-source listening conditions. By combining control with naturalness we aim to better understand what conditions are likely to maximize the success of cochlear implant users. In addition to offering opportunities for translation from research to clinic, the proposed research asks questions that probe auditory mechanisms in normal-hearing listeners and aims to provide knowledge that can advance basic science. These studies are conducted using models that simulate cochlear implant listening. Aim 1 investigates factors at the level of individual electrically pulsed signals. Using a research processor designed to carefully control selected binaural pairs of electrodes, cues that are known to be important for binaural hearing will be introduced to multiple electrode pairs, simulating aspects of real-world listening and testing effects of electrode mismatch across the ears. Aim 2 uses digital signal processing for audio signals. Using a custom made binaural program, clinical speech coding strategies with varying numbers of pitch-matched binaural channels are used. We will aim to demonstrate conditions under which preservation of binaural cues offers advantages for binaural abilities in bilateral CI users that have not been previously seen. Aim 3 is to investigate the effects of age of deafness and age at onset of bilateral hearing on performance. We will systematically recruit patients into prospective groups that vary according to pre-lingual, mid-childhood or adult onset of deafness, with unilateral and bilateral activation ages controlled. We will test hypotheses about auditory plasticity and its impact on binaural hearing. Overall, this project will offer opportunities for translation from research to clinic with important clinical implications in terms of bilateral fitting choices, rehabilitation and counseling. In addition, we will advance basic science in the auditory field by investigating mechanisms in NH listeners under degraded listening conditions.